Optical fibers for use in optical fiber communication systems generally widely available have a structure in which the outer circumferential surface of a core is surrounded by a cladding and optical signals propagate through the inside of this core to transmit information. Nowadays, with the widespread use of optical fiber communication systems, information volumes to be transmitted are tremendously increasing.
In order to achieve such an increase in the transmission capacity of optical fiber communication systems, the transmission of a plurality of signals using a multicore fiber is known. The multicore fiber has a plurality of cores. The outer circumferential surfaces of the plurality of cores are surrounded by one cladding. The plurality of signals is transmitted with light beams propagating through the cores. In order to achieve a further increase in the transmission capacity, transmission using a few-mode multicore fiber (FM-MCF) for multimode communication is also known. In the cores of the multicore fiber, information is superposed on an LP01 mode light beam that is in the fundamental mode, and information is also superposed on light beams in higher modes than the fundamental mode for information communication. Such a few-mode multicore fiber is disclosed in Non Patent Literature 1 below, for example.
Multicore fibers may have crosstalk between cores. The crosstalk is optical transition from cores to the adjacent cores. In the few-mode multicore fiber, this crosstalk is more prone to be noticeable. For example, a so-called step index core having a refractive index approximately constant in the radial direction is widely used in optical fibers. In the step index core, higher mode light beams like an LP11 mode light beam are easily spread to the outer side of the core than the LP01 mode light beam is. Thus, the higher mode light beam has an effective area larger than the effective area of the LP01 mode light beam, easily causing the crosstalk between higher mode light beams propagating through the cores adjacent to each other.
A decrease in the effective area of the higher mode light beam can be considered. However, a decrease in the effective area of the higher mode light beam considerably shrinks the effective area of the LP01 mode light beam, resulting in concern with the degradation of signal quality due to nonlinear effects and an increase in connection loss. Thus, in the design of the few-mode multicore fiber, the crosstalk between higher mode light beams is reduced with no decrease in the effective area of the LP01 mode light beam. An example that satisfies these conditions is a heterogeneous core few-mode multicore fiber disclosed in Non Patent Literature 2 below, for example.    [Non Patent Literature 1] D. Soma et al., “2.05 Peta-bit/s Super-Nyquist-WDM SDM Transmission Using 9.8-km 6-mode 19-core Fiber in C band,” Proc. ECOC2015, PDP3.2, 2015.    [Non Patent Literature 2] K. Shibahara et al., “Dense SDM (12-core×3-mode) Transmission over 527 km with 33.2-ns Mode-Dispersion Employing Low-Complexity Parallel MIMO Frequency-Domain Equalization,” Proc. OFC2015, Th5C.3, 2015.
The heterogeneous core few-mode multicore fiber disclosed in Non Patent Literature 2 above has manufacturing imperfections in which a plurality of types of cores has to be prepared accurately as the cores are designed. The multicore fiber also has imperfections in which optical properties are varied depending on core types. Therefore, a reduction in crosstalk is desired by a method different from a method for the heterogeneous core multicore fiber.